# Capacitor Monitoring

Discussion in 'General Electronics Chat' started by Circuits101, Mar 16, 2011.

1. ### Circuits101 Thread Starter New Member

Feb 24, 2010
11
0
How can I monitor the current charge state of a capacitor bank during charging. I would like to display the amount the capacitor is charged with an LED bar graph and then when it is fully charged stop charging.

2. ### Wendy Moderator

Mar 24, 2008
20,735
2,499
The simple answer is voltage. A cap will charge to the power supply voltage and stop.

It does so using a RC charge curve, which is approx 63% per TC (time constant, TC = R X C).

3. ### Circuits101 Thread Starter New Member

Feb 24, 2010
11
0
Ok so I can just measure voltage across the capacitor similar to a battery and then do a voltage divider to power the LEDs

4. ### russ_hensel Well-Known Member

Jan 11, 2009
818
47
Or get the answer on chiphacker.

5. ### Talguy Member

Aug 17, 2009
14
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yea a capacitor acts similar to a battery.

6. ### studiot AAC Fanatic!

Nov 9, 2007
5,005
513
No a capacitor does not act 'similar to battery' during charge.

A battery will not exceed its peak charge voltage
A capacitor will continue charging towards the charging voltage

So charging a nominal 12v battery from 20 volts will get the battery up to about 14.7 volts.
Continued attempts to charge beyond this may well damage the battery.

Charging a capacitor from 20 volts will quite quickly bring the capacitor pretty close to 20 volts.
Obviously the rates depend upon circuit resistances.

7. ### Jaguarjoe Active Member

Apr 7, 2010
770
90

Use an LED bar graph driver like the LM3914. It will convert a 0-1 volt analog input into 10 distinct LED outputs. You can cascade them to have many more LED's.

http://www.national.com/ds/LM/LM3914.pdf

The easiest way is a good old NE-2 neon lamp and the right resistor.

Maybe you can get some tips from photoflash capacitor monitoring circuits.

Last edited: Mar 17, 2011
8. ### jaclement Active Member

Apr 15, 2009
34
0
If you are chargiing the capacitor bank with a resistor in the positive suppy, you can put an LED and resistor that are in series across that resistor. With the right size current limiting resistor, the LED will glow brightly at the start, and get dimmer as the bank charges. The LED will go out,however before the bank is fully charged

9. ### Wendy Moderator

Mar 24, 2008
20,735
2,499
Or you could use a voltmeter.